No need to patch things. Just change your stylesheet slightly.
The !important in the Kobo rule is evil for a built-in rule, because it overrides anything that isn't similarly marked !important. However, if you add !important to your stylesheet, specificity rules come back into play.
In other words:
Code:
div.prose > p.center {text-align:center !important; text-indent:0 !important;}
That will have greater specificity, and thus should override the built-in stylesheet.